As measuring or control means used for controlling plants or equipment, there have been known a measuring means like a measuring transmitter, a kind of sensor, which is supplied with electric power and converts measurement result such as temperature, humidity, and pressure into current to be outputted; a measuring means such as a sensor like a thermocouple or resistance thermometer bulb, which outputs measurement result as an analog voltage or resistance; and a control means having a contact for switching on/off upon detecting that pressure or temperature reaches a prescribed value and being utilized for detecting ambient conditions.
In such a measurement instrument or a driven object such as an actuator, the instruction side to send instruction to the plant or equipment is insulated from the side performing measurement or driving or controlling in the plant or equipment (hereafter referred to as the plant side) depending on use for the purpose of preventing affections on the human body or evading affections of noise. This is generally done by adopting an insulation transformer for transforming the voltage of the electric power supplied from the power source and insulating a measurement signal sent from the plant side to instruction side from an instruction signal or control signal sent from the instruction side to the plant side by means of a photocoupler, insulation signal relay, insulation amplifier; insulation transformer, etc.
In recent years, there has been an increased demand for performing soundness diagnosis of circuit in the field of instrumentation and measurement for the purpose of raising reliability of the system by confirming soundness of output signals and circuit wiring, that is, by confirming whether instruction signals are transmitted accurate to the control means and whether there is a breaking of wire or short circuit occurred in the circuit.
Block diagrams of FIGS. 10-13 show examples of conventional drive circuits of measuring or control means and those added with a soundness diagnosing circuit.
First, FIG. 10 is a block diagram of a circuit in the case of a sensor such as a measuring transmitter, which is supplied with electric power and outputs measurement results of temperature, humidity, and pressure converting them into analog signals. A voltage of 24V, for example, is applied to a measuring transmitter 100, and electric power is supplied to a signal conversion circuit 108, and modulation circuit 109, from a power source circuit 101.
The power source circuit 101 consists of a pulse generating circuit 103 for converting electric power supplied from a power source 102 into pulse voltage, an insulation transformer 104 for changing the voltage of the pulse voltage from the pulse generating circuit 103 and insulates the plant side from instruction side, a rectifying circuit 105 for rectifying the pulse voltage increased by the insulation transformer 104, and a constant-voltage circuit 106 for smoothing the rectified pulse voltage into a constant voltage.
The signal conversion circuit 108 and modulation circuit 109 are for inputting the analog results measurement of the measuring transformer 10 such as temperature, humidity and pressure as voltage signals to the insulation transformer 110 used for dividing the measurement transmitter 100 side from the instruction side. The measurement transmitter 100 can not input the measurement signals to the insulation transformer 110, because electric currents generated by the temperature, humidity and pressure sensors are currents varying in a range of about 4˜20 mA. Therefore, the measurement results obtained as electric currents can not be inputted as it is in the insulation transformer, so, the current signals are converted into voltage signals by the signal conversion circuit 108, and further converted into alternating voltage signals by the modulating circuit 109 to be inputted to the insulation transformer 110. Then, the output of the insulation transformer 100 is reconverted to a current or voltage signal 112 to be outputted outside as measurement results.
A broken line denoted by reference numeral 107 in FIG. 18 represents an insulation barrier insulating the measuring side (instruction side) from the measuring device side (plant side). In the following explanation, constituent components similar to those in FIG. 10 are denoted by the same reference numerals and detailed explanation is omitted.
FIG. 11 is a block diagram of a circuit in the case of a sensor such as a thermocouple and resistance thermometer bulb, which outputs measurement result as a change in voltage and electric resistance.
The power source circuit 101 consists, similarly to the case of FIG. 11, of a power source 102, pulse generating circuit 103, insulation transformer 104, and rectifying circuit 105, constant-voltage circuit 106. Electric power is supplied to a signal conversion circuit 121 and modulation circuit 122. The voltage signals and resistance signals from thermocouples and resistance thermometer bulb are converted into voltage signals by the signal conversion circuit 121 and modulated by the modulation circuit 122, amplified by an insulation transformer 123 which is a signal insulating means to insulate the plant side from instruction side, then the output from the insulation transformer 123 is reconverted into current or voltage signals 125 by a demodulating circuit 124 to be outputted outside as measurement results.
FIGS. 12 and 13 are block diagrams of circuit in a case of a control means having a contact (hereafter referred to the contact depending on circumstances) for switching on/off upon detecting that pressure or temperature reaches a prescribed value and being utilized for detecting ambient conditions. The electric power source circuit 101 for driving contacts 140 and 143 for outputting ON/OFF signals depending on pressure and temperature consists of an electric power source 102, a pulse generating circuit 103, an insulation transformer 104, a rectifying circuit 105, and a constant voltage circuit 106 similarly to the case of FIG. 10 and FIG. 11. In this way, even when there are a plurality of contacts in the circuit, generally a single electric power source circuit is adopted as a common power source from a viewpoint of cost saving. Signals for switching on/off the contacts 140 and 143 are sent to photocouplers 141 and 144, which are signal insulation means provided to insulate the plant side from the instruction side. Light emitting elements constituting the photocouplers 141 and 144 emit light when the contacts are “ON” respectively, and binary signals 142 and 145 of ON/OFF are outputted to outside from light-sensitive elements respectively to transmit temperature and pressure of the plant side to the instruction side.
In the circuits shown in FIGS. 11 and 12, circuit soundness such as occurrence of a breaking of wire or short circuit can be judged to some extent from conditions that the measurement result is not transmitted to the instruction side or that the measurement result does not vary from a constant value. However, in the circuit shown in FIG. 12, when intending to soundness diagnosis of circuit, it is necessary to confirm whether the control means (contacts) are operating normally in addition to confirm whether there is a breaking of wire or short circuit. To confirm whether the control means is operating normally or not can not be performed without composing the circuit to match the purpose.
Therefore, in the circuit of FIG. 13 in which soundness diagnosis can be performed, the diagnosis is performed by providing a block 146, in which status signal of ON/OFF of the contacts 140, 143 are perceived in an analog fashion using A/D conversion function contained in an microcomputer and evaluated to judge the state of ON/OFF of the contacts, whether there is a short circuit or malfunction of the contacts, and whether there is a breaking of wire or short circuit occurred in the circuit.
The result is sent to the photocoupler 141 which is an insulation means for insulating the plant side from the instruction side, and outputted as a serial communication signal to a communication signal receiving part 147.
As mentioned above, in the conventional circuit, the measuring transmitter 100 and control means (contacts), 140,143, etc. are driven, and insulation interfaces between measuring side and control side are implemented by providing an insulation electric power source 101 of 24V for example which includes the insulation transformer 104 and constant voltage circuit 106 for the purpose of driving the signal conversion circuit, modulating circuit, and microcomputer; an insulation amplifier which includes the insulation transformer 110 or 123; and the photocouplers 141, 144 as insulation means.
Further, when signals from a plurality of contacts are to be received insulated from one another as shown in FIGS. 12 and 13, generally a single electric power source is provided for a plurality of contacts for common use from a point of view of cost saving. Therefore, when the contacts are located at two or more locations remote from one another, voltage differences caused by voltage drop due to difference of length of signal cables has a considerable influence on the circuit system, so, it has been necessary fundamentally to evaluate the state of contacts located near to one another. Furthermore, as a common electric power source is used, signals are evaluated in an analog fashion in the input sides of the insulation means (i.e. the plant side) when a circuit soundness diagnosing function is added to the circuit.
Therefore, there have been problems as follows in the conventional circuits for measurement and control and soundness diagnosing circuit for confirming whether there is a breaking of wire or short circuit:
(A) Circuits for constant voltage, and modulation and demodulation of signals, are needed, which has resulted in increased cost.
(B) It has been usual that a single insulation electric power source is used for a plurality of control means in common because insulation electric power source is expensive. Therefore, when two or more control means are located from each other, voltage difference caused by voltage drop due to difference of length of signal cables has a considerable influence on the circuit system, so, it is necessary fundamentally to evaluate signals of the control means located near to one another.
As to the art for detect braking of wire, there are disclosed for example in patent literature 1 (Japanese Laid-Open Patent Application No. 2006-023105) a method of detecting breaking of wire by applying a pulse signal to the wire, and comparing the current wave shape measured with the reference current wave shape to judge the presence or absence of breaking of wire from difference in both the wave shapes, and in patent literature 2 (Japanese Laid-Open Patent Application No. 2004-198302) a circuit for detecting breaking of wire by applying a pulse signal for checking via an impedance component to the signal wire for detecting breaking of wire, and comparing the signal obtained from the signal wire with the pulse signal for checking to judge the presence or absence of breaking of wire.
As to diagnosis of electric circuits, there is disclosed for example in patent literature 3 (Japanese Laid-Open Patent Application No. 8-005708) a method of diagnosing electric circuits and diagnosing device used for the method. With which conditions of electric apparatuses are diagnosed for the purpose of improving efficiency of diagnosis operation by facilitating measurement record management and further decreasing occurrence of man-caused errors, by reading out information written and stored in a nonvolatile memory concerning measurement results of characteristics or things concerning measurement of the electric apparatuses, or measurement results of characteristics or things concerning measurement of the electric apparatuses, and comparing the read-out information with the information of-the-moment concerning measurement results of characteristics or things concerning measurement of the electric apparatuses.
However, with the art taught in the patent literature 1 and 2, means for applying pulse signals and a memory for memorizing reference current wave shape are needed, and with the electric circuit diagnosing device disclosed in the patent literature 3, a memory memorized information concerning measurement results of characteristics or things concerning measurement of the electric apparatuses is needed, and further a means for measuring characteristics of the circuit and a means, for comparing the measurement result with the reference data, resulting in complicated composition. Therefore, problems cited in the items (A) and (B) can not be solved by these art.